Control network for LED-based lighting system in a transit vehicle

ABSTRACT

A transit vehicle lighting system has a plurality of LED-based lighting fixtures for providing interior illumination. A control network comprises a plurality of slave nodes for controlling the LED-based lighting fixtures, and a master node for controlling the slave nodes. The master node may be connected to the slave nodes by a combined power/communication bus in a daisy chain fashion. The slave node may include a power regulator and a controller for providing a target current command to the power regulator, and may adjust the target current based upon temperature measurements or a recorded age of the LEDs. An optical sensor may provide automatic dimming. A reduced number of LEDs may be used in an emergency mode. The lighting fixture may include a ceiling panel fixture and a riser panel attachable by way of a hinge mechanism.

RELATED APPLICATION INFORMATION

This application is a utility application claiming the benefit of U.S.Application Ser. No. 60/966,547 (formerly Ser. No. 11/779,260), filed onJul. 17, 2007, hereby incorporated by reference as if set forth fullyherein.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The field of the present invention generally relates to lighting systemsand, more particularly, to control networks for lighting systems used intransit vehicles or conveyances such as buses, lightrail cars, and thelike.

2) Background

Transit vehicles and similar conveyances typically rely on fluorescentlighting to provide illumination, in part because of the historicalefficiency of such lighting as compared to, e.g., incandescent lighting.However, fluorescent lighting has drawbacks and limitations.

For example, fluorescent lighting has a relatively short lifetime and ahigh current draw as compared to some other types of lighting, inparticular low power light-emitting diodes (LEDs). In a typical setting,the fluorescent lights receive power from the transit vehicle's battery,which is usually 24 volts, the output of which is “chopped” to providean alternating current source. The nature of fluorescent lights leads tohigh electro-magnetic fields, making compatibility with other electricalequipment challenging. Fluorescent lights also often have problems witharcing, which in turn can pose fire dangers or else blow out electricalcomponents and/or cause power ripples. Fluorescent lights are alsotraditionally wired in a point-to-point fashion, requiring a substantialamount of wiring.

In addition, fluorescent lighting is not very amenable to flexiblecontrol, and can be difficult to dim. For example, if the fluorescentlamp filaments get too cool when attempting to dim the lamp, then thelamp may simply go out. If current continues to flow while theelectrodes are at an improper temperature, then severe rapid degradationof the filaments is possible.

Some efforts have been made to try to incorporate LED lights in transitvehicles, but it is not clear that such efforts have been able toovercome the many disadvantages and limitations of fluorescent lighting.

It would therefore be advantageous to provide a lighting system for atransit vehicle or conveyance, which overcomes one or more of thedisadvantages or limitations of conventional lighting systems. It wouldfurther be advantageous to provide such a lighting system which hasimproved lifetime and power/current consumption, requires less wiring,and/or is more easily controlled. It would also be advantageous toprovide such a lighting system which is inexpensive and not overlycomplex to implement or deploy, and/or has other benefits and advantagesnot found in conventional transit vehicle lighting systems.

SUMMARY OF THE INVENTION

The invention is generally directed in one aspect to a novel andversatile lighting system that is particularly well suited for a transitvehicle or similar conveyance, but which may find other uses orapplications as well. According to one embodiment as disclosed herein,an LED-based lighting fixture for a transit vehicle comprises aplurality of LEDs for illuminating an area of a transit vehicle, a powerregulator receiving an input power supply signal, and a controller forproviding a target current command to the power regulator. Thecontroller may adjust the target current based upon temperaturemeasurements received from a temperature sensor, and/or based upon arecorded age of the LEDs. An optical sensor may provide automaticdimming. A reduced number of LEDs may be employed when in an emergencymode.

In various forms and embodiments, the lighting fixture may be part of anLED-based control network in which a plurality of LED-based lightingfixtures each comprise a plurality of LEDs for illuminating an area of atransit vehicle, a plurality of slave nodes for controlling theLED-based lighting fixtures, and a master node in communication with theslave nodes. A digital communications bus may be provided forcommunicating among the master node and the slave nodes. The digitalcommunications bus may be combined with a power supply cable, and themaster node may be connected to the slave nodes by wiring the combineddigital/power cable/bus in a daisy chain fashion from slave node toslave node.

In other embodiments, the LED-based lighting fixture may include aceiling panel fixture having a ceiling panel adapted to abut a transitvehicle interior sidewall and an LED lighting mount on which the LEDsare disposed, and a riser panel attached to the ceiling panel fixtureand adapted to abut a transit vehicle ceiling or interior top mountingsurface. The riser panel may include a locking mechanism for securingthe riser panel to the transit vehicle ceiling or interior top mountingsurface, and a detachable hinge for releasably attaching to the ceilingpanel fixture, whereby the riser panel may be fully removed from andreattached in position by releasing the locking mechanism.Alternatively, the riser panel may include a permanent hinge forattaching the riser panel to the ceiling panel fixture, whereby theriser panel may be rotationally swung open by releasing the lockingmechanism.

Further embodiments, variations and enhancements are also disclosedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing an example of a lighting controlsystem for a transit vehicle in accordance with one embodiment asdisclosed herein.

FIG. 2A is a high level architectural diagram of an embodiment of alighting control system in accordance with the principles illustrated inand/or described with respect to FIG. 1.

FIG. 2B is a diagram of an embodiment in which LED-based lightingfixtures are connected in series, according to one example of a lightingcontrol system constructed in accordance with various principlesillustrated in and/or described with respect to FIG. 2A.

FIG. 3 is a block diagram of a master node as may be used, for example,in the lighting control system depicted in FIG. 2A or otherwise.

FIG. 4 is a block diagram of an LED-based lighting fixture and slavenode as may be used, for example, in the lighting control systemdepicted in FIG. 2A or otherwise.

FIG. 5 is an architectural diagram of a master-slave control network fora transit vehicle, illustrating a possible relationship of the lightingcontrol system to other components in the control network.

FIG. 6 is a block diagram of a network node as may be used within thecontrol network architecture shown in FIG. 5 or otherwise.

FIG. 7 is a diagram of a hierarchical control network architecture inaccordance with another embodiment, and having multiple second-tierbuses.

FIG. 8 is an architectural diagram of another embodiment of ahierarchical control network, having a third-tier bus, as may be used inconnection with a lighting system control network as disclosed herein.

FIG. 9 is an architectural diagram of a hierarchical control networkwith a matrix architecture providing supervision of mid- and lower-tierlevels of the control network, as may be used in connection with alighting system control network as disclosed herein.

FIGS. 10A and 10B are timing diagrams for communication in ahierarchical control network having a matrix architecture, such as thecontrol network illustrated in FIG. 9.

FIGS. 11A and 11B are diagrams illustrating cross-sectional views of alighting panel fixture with LEDs for illumination as may be used in atransit vehicle, while FIGS. 11C through 11G are diagrams illustratingoblique views from different angles of the lighting panel fixture orportions thereof illustrated in FIGS. 11A and 11B.

FIG. 12 are graphs illustrating an example of voltage and currentwaveform characteristics of an array of LEDs controlled by a lightingcontrol system as disclosed herein.

FIG. 13A is a chart illustrating an example of LED current relative toage (in months), and FIG. 13B is a graph illustrating similarinformation.

FIGS. 14A and 14B are examples of current derating graphs illustratingin general a drop in maximum LED current rating with an increase inforward junction temperature.

FIGS. 15A and 15B are diagrams illustrating, respectively, front andback oblique views of another embodiment of a lighting panel fixturewith LEDs for illumination as may be used in a transit vehicle, whileFIGS. 15C and 15D illustrate cross-sectional side views (the latterbeing an assembly diagram) of the lighting panel fixture illustrated inFIGS. 15A and 15B. FIG. 15E is a variation having a permanent hinge inthe curved riser panel for access behind the lighting panel fixture.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

According to various embodiments as disclosed herein, a lighting fixtureand control network for an LED-based lighting system for use in atransit vehicle may comprise, for example, one or more of (i) aplurality of lighting fixtures each having LEDs for illuminating an areaof a transit vehicle; (ii) a power regulator for receiving an inputpower supply signal; (iii) a controller for providing a target currentcommand to the power regulator; (iv) a plurality of slave nodes forcontrolling the LED-based lighting fixtures; (v) a master node incommunication with the slave nodes; (vi) a digital communications busfor communicating among the master node and the slave nodes; (vii) apower supply cable integrated with the digital communications bus,connecting the master node to the slave nodes in a daisy chain fashion;(viii) an optical sensor for detecting ambient light in order to provideautomatic dimming of the LEDs; and (ix) an emergency mode in which areduced number of LEDs are employed. In certain embodiments, thecontroller may adjust the target current based upon temperaturemeasurements received from a temperature sensor, and/or based upon arecorded age of the LEDs. In other embodiments, the LED-based lightingfixture may include a ceiling panel fixture having a ceiling paneladapted to abut a transit vehicle interior sidewall and an LED lightingmount on which the LEDs are disposed, and a riser panel attached to theceiling panel fixture and adapted to abut a transit vehicle ceiling orinterior top mounting surface. The riser panel may include a lockingmechanism for securing the riser panel to the transit vehicle ceiling orinterior top mounting surface, and a hinging mechanism (detachable orpermanent) for attaching to the ceiling panel fixture, whereby the riserpanel may be fully removed from and reattached in position by releasingthe locking mechanism or else rotationally swung open by releasing thelocking mechanism.

FIG. 1 is a conceptual diagram of an example of a preferred lightingcontrol system as may be used in connection with a transit vehicle, inaccordance with one embodiment as disclosed herein. In FIG. 1, apreferred lighting control system 100 comprises part of, or interactswith, a vehicle control network 109 that controls various functions andoperations of a transit vehicle 149. The vehicle control network 109,through various network nodes or other components, may control ormonitor aspects of the transit vehicle's operational features such as,e.g., its transmission system, engine sensors or controls, airconditioner, light indicators or displays, front and rear doors,magnetic breakers, kneeling mechanism, wheel chair platform, headlights,tail lights, control switches, instrument panel, internal lightingsystem, and so on. In one aspect, the lighting control system 100 maycomprise a sub-system of the vehicle control network 109, although inother embodiments the lighting control system 100 may be standalone orindependent of the main vehicle control network 109.

As further illustrated in FIG. 1, power may be provided to the vehiclecontrol network 109 and/or lighting control network 100 via an on-boardbattery 118, which typically will be rated 24 Volts for a transitvehicle although the actual voltage output may vary over time and undervarious conditions. The lighting control network 100 in this examplecomprises a plurality of network nodes including a master node 103 and aseries of slave/LED nodes 102, each of which constitutes or isassociated with an LED-based lighting fixture having local electroniccontrol but responsive to the master node 103. A power/data bus 104preferably interconnects the series of slave/LED nodes 102 in adaisy-chain fashion. Each of the slave/LED nodes 102 may include aplurality of LEDs 107 arranged on a lighting panel or other fixture toprovide illumination for a particular interior region of the transitvehicle 149.

FIG. 2A is a high level architectural diagram of an embodiment of alighting control system 200 in accordance with the principlesillustrated in FIG. 1, showing additional details thereof. Asillustrated in FIG. 2A, the lighting control system 200 comprises amaster node 203 and a series of LED-based lighting fixtures 202, each ofwhich constitutes or is associated with a slave/LED network node havinglocal control electronics responsive to the master node 203, similar tothe lighting control system 100 of FIG. 1. Each of the LED-basedlighting fixtures 202 in this example comprises a network (slave) node223 and one or more LED units 224. Each LED unit 224 comprises one ormore LEDs arranged thereon to provide illumination for a particularinterior region of a transit vehicle. The LED-based lighting fixtures202 are preferably interconnected in series, in a daisy-chain manner, asdescribed with respect to FIG. 1. A power bus comprising a power supplyline 211 from the vehicle battery and a ground line 213 is preferablyconnected from the master node 203 to each LED-based lighting fixture202 and slave node 223 thereof, in series. Likewise, a command/data bus212 may be connected from the master node 203 to each LED-based lightingfixture 202 and slave node 223 thereof, in series. Although othertechniques may be used, by connecting the LED-based lighting fixtures indaisy-chain fashion, wiring of the interior lights for the transitvehicle may be advantageously simplified.

In operation, the master node 203 may communicate with the main vehiclecontrol network (e.g., control network 109 in the example of FIG. 1) andmay respond to commands or queries therefrom, and/or may send alarmsignals, interrupts, or status information to the main vehicle controlnetwork. The master node 203 may also control the various LED-basedlighting fixtures 202, and slave nodes 223 thereof, through commands orqueries transmitted over the command/data bus 212. The command/data bus212 may be embodied as any suitable bus, and may, for example, be serialor parallel in nature, and may be electrical or optical in nature (e.g.,using fiber optics). The master node 203 and slave nodes 223 may bearranged in a linear configuration, or else may be arranged in a loopconfiguration as explained hereinafter for increased reliability andredundancy. Commands or inquiries may be transmitted from the masternode 203 over the command/data bus 212 to the first slave node 223(Slave Node 1), which then propagates the commands or inquiries over thecommand/data bus 212 to the next slave node 223 (Slave Node 2) in thechain, and so on in sequence until the command or inquiry reaches thelast slave node 223 (Slave Node N) if necessary. Commands or inquiriesneed not be propagated if the targeted slave node 223 receives thecommand or inquiry and responds thereto; however, it may nonetheless bedesirable in some configurations to propagate all commands andinquiries, or certain subsets thereof, to all of the slave nodes 223 inthe lighting control network 200.

Via commands or inquiries conveyed over the command/data bus 212, themaster node 203 may control, individually or by group, the individualLED units 224 of each LED-based lighting fixture 202. The master node203 may, for example, command certain LED units 224 to turn on, turnoff, dim by a specified amount, intensify by a specified amount, flash(e.g., in emergency situations), or activate according to apredetermined pattern. Dimming may be accomplished by pulse widthmodulation, thereby reducing the percentage of time that the LEDs areturned on, and/or by turning off selected LEDs and thereby reducing thenumber of “on” LEDs at a given time. Likewise, the intensity of lightmay be increased by increasing the percentage of time that the LEDs areturned on using pulse width modulation, and/or by turning on additionalselected LEDs which are initially in an “off” state. The master node 203may convey commands to the slave nodes 223 at each LED-based lightingfixture 202, and, in response thereto, the slave nodes 223 may beresponsible for locally controlling the electronics at each LED-basedlighting fixture 202 in order to implement the command received from themaster node 203.

FIG. 2B is a diagram of an embodiment in which LED-based lightingfixtures 252 are connected in series, according to one example of alighting control system 250 constructed in accordance with variousprinciples illustrated in and/or described with respect to FIG. 2A. InFIG. 2B, the lighting control system 250 includes a plurality ofLED-based lighting fixtures 252, each of which includes, among otherthings, one or more LED units 274, as may be embodied according to thevarious examples described herein. The LED units 274 typically comprisea plurality of LEDs for illumination of the interior of a transitvehicle. The LED-based lighting fixtures 252 are illustrated in FIG. 2Bas physically connected in series. They may, for example, be physicallyinstalled along the ceiling region above the seating area of a transitvehicle, thereby providing continuous illumination for occupants in theinterior of the transit vehicle. Each of the LED-based lighting fixtures252 also preferably includes a slave node 273 as described with respectto FIG. 2A and other embodiments elsewhere herein. The slave nodes 273of the LED-based lighting fixtures 252 may, as illustrated in FIG. 2B,be connected by a series of cables 279, thereby simplifying wiringrequirements of the transit vehicle lighting system. Complementarymating connectors (e.g., female and male connectors 281, 282) may beused to secure the cables 279 to one another between the various slavenodes 273. Each cable 279 may carry a plurality of electrical signalsincluding, e.g., battery power and ground signals (211 and 213 in FIG.2A) and one or more command/data bus signals (212 in FIG. 2A).

As generally described previously, the slave nodes 273 provide localcontrol of the various LED units 274 on a particular LED-based lightingfixture 252. The slave nodes 273 are accordingly coupled to the variousLED units 274 via one or more control and/or power signals 285. Apreferred set of particular signals that are included among the controland/or power signals 285 is described with respect to FIG. 4 herein,although not all the signals in FIG. 4 may be necessary in variousembodiments, or else additional electrical signals may be provided ifadditional functions are included.

As noted, one potential benefit of the arrangement in FIG. 2B is thesimplification of wiring needed for the illumination system in a transitvehicle. Such an arrangement dispenses with the need for individualpower cables being separately routed from the battery source to eachlighting fixture (as is typically done with fluorescent lightingfixtures), and also, by virtue of using LEDs, dispenses with the needfor ballasts for each of the lighting fixtures. The arrangement in FIG.2B therefore uses less wiring, and consequently can be less expensiveand more lightweight. Use of serial cables 279 also makes wiring ofpower and control signals relatively simple and easy to install andmaintain.

FIG. 3 is a block diagram of a preferred master node 300 as may be used,for example, in the lighting control system depicted in FIG. 2A. In theexample illustrated in FIG. 3, the master node 300 is connected to thepower supply line 211 and ground line (shown as separate input andoutput ground lines 213 a and 213 b, collectively referred to as groundline 213). The master node 300 preferably includes a voltage regulator308 which receives a power signal from the power supply line 211, andconverts the (typically 24 Volt) input power signal to a stable lowvoltage power output signal for digital electronics and other componentswithin the master node 300. The voltage regulator 308 outputsappropriate internal power supply signals 328, 329 and 330 to othercomponents of the master node 300.

The master node 300 also preferably includes a micro-controller 320 forperforming the supervisory control and logic functions of the lightingcontrol system. Although not expressly shown in FIG. 3, themicro-controller 320 would be understood to include appropriate memory(e.g., RAM and/or ROM), interfaces, and drivers, as well as logiccircuitry including, for example, a microprocessor, FPGA, or otherprogrammable logic. A main network command line driver 305 interfaceswith the main vehicle control network via a network communication bus215. Likewise, a lighting control system command line driver 315interfaces with other lighting control system network components via adata/command bus 212, which is interconnected to downstream LED-basedlighting fixtures in a manner described with respect to FIG. 2A.

In operation, the micro-controller 320 communicates with the mainvehicle control network over the network communication bus 215 and may,for example, send periodic status updates or alarm signals to the mainvehicle control network, or else respond to various commands orinquiries received from the main vehicle control network. Themicro-controller 320 also is responsible for communicating with theslave nodes 223 of the LED-based lighting fixtures 202 via commands orinquiries sent over the data/command bus 212, and may also, for example,receive periodic status updates or alarm signals sent by the slave nodes223. The master node, under control of micro-controller 320, maycommunicate with the slave nodes according to, e.g., a polling schemewhereby the slave nodes 223 are periodically queried for statusinformation or other data.

FIG. 4 is a block diagram of a preferred LED-based lighting fixture 400,including a slave node 423, as may be used, for example, in the lightingcontrol system depicted in FIG. 2A. In the example illustrated in FIG.4, the LED-based lighting fixture 400 is conceptually divided into aslave node 423, which forms a network node of the lighting controlnetwork, and one or more LED units or modules 424 as generally describedpreviously with respect to FIG. 2A. The slave node 423 of the LED-basedlighting fixture 400 is connected to an incoming power supply line 411 aand ground line 413 a received from an upstream LED-based lightingfixture 400 in the daisy chain (as shown in the arrangement of FIG. 2A),or, if the first fixture in the chain, from the master node 203. TheLED-based lighting fixture 400 preferably includes a voltage boostregulator 408 which receives a power signal from the power supply line411 a, nominally at 24 Volts for a typical transit vehicle (although theactual voltage may swing significantly), and converts the input powersignal to both a stable low voltage power output signal for digitalelectronics and other components within the slave node 423, and aregulated boosted voltage level for the LEDs on the various LED units424. The voltage regulator 408 outputs appropriate internal power supplysignals (not shown) to other components of the slave node 223, such asmicro-controller 420 and any other components requiring a low voltagesource.

The micro-controller 420 performs the local control and logic functionsrequired by the lighting control system for causing the LEDs of the LEDunits 424 to turn on and off, dim, flash if necessary, and so on.Although not expressly shown in FIG. 4, the micro-controller 420 wouldbe understood to include appropriate memory (e.g., RAM and/or ROM),interfaces, and drivers, as well as logic circuitry including, forexample, a microprocessor, FPGA, or other programmable logic. Themicrocontroller 420 receives commands or inquiries over a data/commandbus 412 a from an upstream slave node 423 or the master node 203, andalso propagates commands and inquiries as necessary to downstream slavenodes 223 via a continued segment of the data/command bus 412 b.Although not expressly shown in FIG. 4, the slave node 423 would beunderstood to include command line drivers for sending appropriate dataor command signals over the data/command bus 412 a, 412 b.

The LED unit 424 preferably includes a temperature sensor 430 andoptionally an optical sensor 432, the purpose of which will be describedin more detail hereinafter. The temperature sensor 430 and opticalsensor 432 are preferably coupled to the micro-controller 420. Themicro-controller 420 may be provided with access to one or more lifetimecounters 429, which store an indication of the age of the LEDs of theLED units 424 and, in certain embodiments, may affect the amount ofcurrent provided to the LEDs as further described herein.

The number of LED units or modules 424 of the LED-based lighting fixture400 may vary depending upon architectural needs. As one example, theLED-based lighting fixture 400 may include five LED units or modules 424each having, e.g., eight LEDs, for a total of 40 LEDs on a singleLED-based lighting fixture 400. LED protection circuits 443 may beprovided in parallel with each of the LEDs 442, as known in the art, sothat failure of a single LED will not necessarily impede the performanceof the remaining LEDs 442. The LED units 424 are configured in parallel.For a configuration having eight LEDs 442 per LED unit 424, the voltageboost regulator circuitry 408 would provide an output 435 ofapproximately 35 Volts, which some variation depending upon the currentdraw and operational temperature of the LEDs 442. The LEDs 442 arepreferably white in color temperature and have relatively high output.In one embodiment, the LEDs 442 are Luxeon K2 emitters available fromPhilips Lumileds Lighting Company.

In addition to communicating with upstream or downstream nodes of theLED control network, the micro-controller 420 is also preferablyconfigured to control the local electronics to provide the LED units 424with a regulated current appropriate to the age of the LEDs 442, ambientconditions, and incoming control commands (such as dimming commands).The micro-controller 420 may receive commands from the master node 203and, based thereon, set or modify target parameters utilized by thecurrent control circuitry 421 and/or voltage boost regulator circuitry408. For example, the micro-controller 420 may receive a command to dimthe lighting by a specified amount, and in response thereto, themicro-controller 420 would lower or adjust the target parametersutilized by the current control circuitry 421 and/or voltage boostregulator circuitry 408 accordingly. In a preferred embodiment, each LEDunit 424 can be separately controlled and thus individually turnedon/off or dimmed.

While a variety of factors or conditions may cause the LED output tovary over time, the overall output of the LED fixtures may need to meetthe requirements set for mass transit vehicles regardless of ambientconditions or other variable factors. A typical mass transit vehiclestandard might require, for example, 15 foot-candles of illuminationover each bus seat, and 10 foot-candles of illumination in the busaisles. The variable nature of LEDs can make it challenging to meet theminimum brightness standards. The circuitry used for LED-based lightingfixture 400 may be particularly advantageous in this regard.

In one embodiment, the current control circuitry 421, by itself or undercontrol of the micro-controller 420, controls the amount of LED currentbased upon factors including the operational temperature of the LEDs 442and the age of the LEDs 442. The lifetime counter(s) 429 may bereferenced to adjust the duty cycle based on the age of the LEDs 442.The age of the LEDs 442 may be stored in one or more lifetime counters429 and continuously updated based on the micro-controller's internalclock or date stamp. For example, lifetime counter(s) 429 may store anumeric value indicating the age of the LEDs 442 in terms of months ofoperation. The numeric value may be initialized to zero when the LEDunits 424 are installed or replaced, and gradually will be incrementedover time based upon the internal clock or date stamp of themicro-controller 420. In general, the older the LEDs 442, the higher theLED current that may be necessary to achieve the same level ofbrightness. Therefore, the micro-controller 420 may increase the targetcurrent value 422 provided to the current control circuitry 421 overtime as the LEDs age. A chart depicting an example of how the LEDcurrent may be increased over time is illustrated in FIG. 13A. In thisexample, each additional three months of LED age would cause the amountof necessary current to increase by the specified amount. A diagramshowing a portion of this same information in graphical form is shown inFIG. 13B. The micro-controller 420 preferably stores informationrepresenting a chart similar to FIG. 13A or 13B (in the form of, e.g.,an internal table) so that the target current 422 can be readilyadjusted in response to the recorded age of the LEDs 442 as reflected bythe numeric value stored in the lifetime counters 429. The chart in FIG.13A also illustrates the pulse width modulation duty cycle (in terms ofpercentage) that may be used to achieve the specified LED current foreach entry. Thus, the current control circuitry 421 in this example maybe configured to start with relatively low PWM duty cycle (e.g., 43%)when the LEDs 442 are new, and gradually increase the duty cycle overlifetime of LEDs 442 to maximum duty cycle of, e.g., 93% at 150 months.

The operational temperature of the LEDs 442 may be periodically orcontinuously monitored using temperature sensor 430, and thisinformation may also be provided to micro-controller 420.Micro-controller 420 may then determine a target current setting forcurrent control circuitry 421 based on predefined temperature/currentderating information stored in local memory accessible to themicro-controller 420. Examples of current derating curves showing themaximum forward LED current versus operational temperature areillustrated in FIGS. 14A and 14B, for white and colored LEDsrespectively. Generally, the higher the junction temperature of the LEDs442, the lower the maximum safe current setting. If the maximum safecurrent setting is exceeded, the expected lifetime of LEDs 442 can beseriously affected. Information describing the applicable currentderating curve for LEDs 442 may be stored in local memory and utilizedby micro-controller 420 to determine the maximum current setting forLEDs 442 to be applied by current control circuitry 421. In general, solong as the LED junction temperature is below the maximum permissiblelevel, no temperature adjustment will be necessary, but as the maximumpermissible temperature is exceeded the current control circuitry 421will limit the LED target current level 422.

In alternative embodiments, current control circuitry 421 directlyutilizes the values from the temperature sensor 430 and/or lifetimecounters 429, without the need for the micro-controller 420 as anintermediary.

The current control circuitry 421 outputs a control signal to thevoltage boost regulator circuitry 408, whereby the boosted or regulatedvoltage output 435 is applied to the LED arrays 441 of LED units 424. Asthe temperature of the LEDs 442 increases, the current control circuitry421 adjusts the current setting so as to avoid going above the maximumsafe current limit, as described above. FIG. 12 illustrates an exampleof a current waveform characteristic for the LED arrays 441. As shown inFIG. 12, while the regulated voltage from voltage boost regulatorcircuitry 408 may vary over time, the LED current is pulse widthmodulated, and duty cycle set, according to the target current level asset by the current control circuitry 421. The LED current level staysrelatively constant across pulses. As the LED temperature increases, thecurrent level is decreased, under management of the current controlcircuitry 421, in accordance with a characteristic such as reflected inthe chart of FIG. 14A. The current control circuitry 421 preferablymonitors the LED current through a current sense signal 436 (whichpreferably senses the peak current level), in order to keep the currentstable despite possible fluctuations in the regulated voltage level. Thepeak current value and duty cycle information allow the area of graph inFIG. 12 to be determined, which in turn allows determination of theaverage current. The average current may then be compared to the targetcurrent level 422 to maintain the desired current level and adjust theduty cycle as necessary. The current control circuitry 421 may implementa hard current limit, such as 350 milliamps for example, which willdepend on the particular LED specifications.

In one aspect, temperature feedback is used (via temperature sensor 430)in order to adjust the LED current if necessary via current controlcircuitry 421, in a manner which can advantageously extend the lifetimeof the LEDs by minimizing the risk of over-current conditions. Othercontrol information or parameters, such as the measured LED currentlevel, LED age (stored in counter(s) 429), and state of optical sensor432 may also be used to affect the LED current, by altering the dutycycle provided by current control circuitry 421.

In some embodiments, a single temperature sensor 430 may be used for allof the LED units 424. In such a case, the temperature measured from oneLED unit 424 is deemed to be an accurate estimate of the temperature onthe other LED units 424. In other embodiments, a temperature sensor 430may be provided for each LED unit 424.

In some embodiments, the micro-controller 420 provides a range ofcapabilities and useful features relating to the LED-based lightingfixture 400. For example, the micro-controller 420 may respond tocommands from the master node 203 to shut off all or selected ones ofthe LED units 424. A master light shutoff command may also be sent fromthe master node 203 instructing all LED/slave nodes to turn off. Themicro-controller 420 may also respond to commands from the master node203 to dim the LEDs 442 on all or selected ones of the LED units 424, inwhich case the micro-controller 420 may instruct the current controlcircuitry 421 to reduce the PWM duty cycle for the affected LED units424 by an appropriate amount. This feature may be useful, for example,to provide driver-controlled manually adjustable dimming of the lightsin the transit vehicle. The micro-controller 420 may also be operativeto receive an indication of the ambient light from an optical sensor432, and adjust the power of the LEDs 442 in response thereto, therebyproviding an auto-brightness function. For example, the micro-controller420 may have a “day” mode and “night” mode, with the LEDs 442 being setto brighter (i.e., higher duty cycle) in the “night” mode. Themicro-controller 420 may store a threshold value, against which themeasured ambient light is compared in order to determine whichbrightness mode should be applied. More than two possible brightnesssettings responsive to the optical sensor 432 may also be provided. Byreducing the brightness of the LEDs 442 when full brightness is notneeded, the lifetime of the LEDs 442 may be usefully enhanced. Only asingle optical sensor 432 may be needed per transit vehicle; however,more than one optical sensor 432 may be used (for redundancy) and/or atleast one optical sensor 432 may be provided for LED-based lightingfixture 400.

The micro-controller 420 may also provide an overheated warning messageor indication based on the temperature detected by temperature sensor430. A special overheat indicator (which may be a colored LED located ondriver's control panel or display, or locally near the LED-basedlighting fixture 400) may be configured to light up or blink if the LEDs442 overheat.

The LED-based lighting fixture 400 may also usefully provide anemergency exit lighting function. In case of an emergency, the LED-basedlighting fixture 400 may be configured, under control ofmicro-controller 420, to automatically light up to provide passengerswith a visual view and guide them to the nearest exit. A backup powersupply (e.g., rechargeable battery) is preferably included to provideemergency power to the LEDs 442 in this situation. Also, a reducednumber of LEDs 442 can be activated in an emergency situation, so as tominimize the size and rating requirements of the backup power supply.For example, half or fewer of the LEDs 442 could be activated in anemergency situation.

The LED-based lighting fixture 400 may also have a self diagnosticsindicator, which shows if a malfunction has occurred on an LED unit 424or elsewhere in the LED-based lighting fixture 400. In case of amalfunction, a diagnostics message with the details of the failure maybe logged to the system, i.e., sent to the master node 203. Customerscan thereafter access the diagnostics by downloading the informationfrom the master node 203.

Additional features and functions pertain to a preferred mechanicalconfiguration of an LED lighting fixture in accordance with variousembodiments as disclosed herein. One exemplary embodiment is depicted inFIGS. 11A through 11G. FIGS. 11A and 11B are cross-sectional views of alighting panel fixture 1100 with LEDs for illumination, as may be usedin a transit vehicle, while FIGS. 11C through 11G are various obliqueviews of the lighting panel or portions thereof illustrated in FIGS. 11Aand 11B. Other embodiments are illustrated in FIGS. 15A through 15E,described later herein.

As illustrated first in FIG. 11A, the lighting panel fixture 1100includes a ceiling panel 1102, an LED lighting mount 1104, and a riserpanel 1122. The ceiling panel 1102 is generally configured to bepositioned directly over the seating in the transit vehicle or similarconveyance, and may abut the interior sidewall 1140 of the transitvehicle. The riser panel 1122 is, in this particular embodiment, curvedin approximately a quarter arc shape, and would generally reach to theceiling 1130 of the transit vehicle or some other top mounting surface.The riser panel 1122 may optionally be configured to allow mounting ofadvertising prints or similar materials, as is common in the transitindustry. In the embodiment illustrated, the riser panel 1122 isdetachably hinged, such that it can be pulled down (as illustrated inFIG. 11B) and temporarily removed to allow access to, e.g., a utilitypanel or a storage compartment. The LED lighting mount 1104interconnects the ceiling panel 1102 and the riser panel 1122, and theLEDs 1105 (shown in side view) may be positioned along the LED lightingmount in an array or other configuration. A removable light permeablecover 1120 may be provided or affixed to the LED lighting mount 1104,for softening or evening out the illumination from the LEDs 1105. Thelight permeable cover 1120 is preferably of sufficient opaqueness tomake the LEDs 1105 appear as a single broad light source (as opposed todiscrete or individual point light source), without substantiallyaffecting the amount of illumination output from the lighting unit.

FIGS. 11A and 11B also show further details of one possible detachablehinge mechanism for the LED lighting fixture 1100. In this example, arounded hinge member 1124 is generally hook-shaped and configured to fitsnugly in a half-cylindrical shaped hinge clasp 1123, as illustrated inFIG. 11B. A locking member 1126 on the opposite end of the riser panel1122 preferably provides a means to securely attach to the ceiling orsimilar top mounting surface when the riser member is in an uprightposition, as in FIG. 11A. The locking member 1126 may, for example, havea removable protective cover 1129 (as shown in FIG. 11B) and a sleeve1127 for guiding a screw 1128 into a receptor located in the ceiling1130 of the transit vehicle or other conveyance. A protective panelcover 1132 may be positioned on the back of the riser member 1122 foradditional support or reinforcement.

In one embodiment, the LED lighting mount 1104 is constructed usingaluminum extrusion, and operates as a heat sink in addition to providingphysical support for the LEDs. To further facilitate heat dissipation,the LED lighting mount 1104 may be configured with metallic heatdissipating fins 1163 on the backside of one or more of its walls. Suchheat dissipating fins 1163 may be particularly advantageous where highpower LEDs are utilized (typically having operating current from 200 mAto 700 mA or more). The riser panel 1122 may be constructed of alightweight durable material such as fiberglass, or PVC plastic. Ceilingpanel 1102 is preferably formed of a composite material, such as twometal layers sandwiching an insulator material. For example, thecomposite material may be DIBOND™, a lightweight yet rigid aluminiumcomposite material consisting of a polyethylene core and aluminium coversheets with a thickness of approximately 0.3 millimeters, coil-coated onfront and reverse sides according to ECCA-Standards with a protectivefilm. DIBOND material is commercially available from Alcan Inc. ofMontreal, Canada, and is commonly used on transit vehicles. This type ofcomposite material helps prevent condensation when air conditioning isemployed.

The diagrams in FIGS. 11C through 11G show additional views and detailsof the lighting panel fixture 1100 or portions thereof illustrated inFIGS. 11A and 11B. FIG. 11C illustrates an oblique view of the lightingpanel fixture 1100. FIG. 11D illustrates primarily the LED lightingmount 1104 of the lighting panel fixture 1100, showing the opaque cover1120 and various LED modules 1150 each having, in this example, an arrayof multiple LEDs 1105. The length of the lighting panel fixture 1100 mayvary depending upon lighting needs. In one particular example, thelighting panel fixture 1100 (and hence the LED lighting mount 1104) isapproximately 60″ in length. The lighting panel fixture 1100 maycomprise multiple LED units (or “light bars”) 1150, e.g, five LED units1150, each 12″ long. Each LED unit 1150 in this example has eight LEDs1105, arranged in a linear array, as more clearly shown in FIG. 11G(with light permeable cover 1120 removed). The example in FIG. 11G showsfive LED units 1150 each having eight LEDs 1105. The LEDs 1105 may bemounted on a metal PCB for convenient modular attachment and/or to helpdissipate heat. A removable snap-in lens holder 1142 may be provided toallow rapid removal of the light permeable cover 1120.

In a typical transit vehicle (such a passenger bus), six lighting panelfixtures 1100 may be employed on each side of the vehicle. Each LEDlighting panel fixture 1100 may have a control module to drive the LEDs1105 of the multiple LED units 1150, as described with respect to FIGS.2A through 4 previously. The control module may be contained in acontrol module housing 1160 on the back of the LED lighting mount 1104,as illustrated in FIGS. 11E and 11F. Multiple lighting panel fixtures1100 may be connected using cables with complementary connectors ateither end (similar to FIG. 2B), in a daisy chain fashion, with a maleconnector at one end of the lighting panel fixture 1100 and a femaleconnector on the other end. A connector socket 1161 may be provided oneach side of the control module housing 1160 to facilitate suchinterconnection. The complementary connectors may be, e.g., pigtailconnectors and may have, in one embodiment, three pins (power, ground,and a serial bus for communication) thereby conveying three electricalsignals.

In the example illustrated in FIGS. 11A through 11G, the conduits inwhich the LED units 1150 reside are open at either end, so that when theLED lighting panel fixtures 1100 are placed in a row, a singlecontinuous conduit is formed. By contrast, fluorescent lights generallyrequire sockets at each end, and thus, in conventional transit vehiclelighting systems, a continuous lighting conduit does not exist. Having asingle continuous conduit using a series of the lighting panel fixtures1100 can significantly ease cleaning of the lighting fixtures. Forexample, it is possible to vacuum or force air through the entirecontinuous lighting conduit, providing the possibility of aself-cleaning mechanism. As one possible example, a bypass valve may beprovided to the air conditioning or ventilation system whereby forcedair can be selectively directed from one end of the continuous lightingconduit and exited at the other end, facilitating cleaning of thelighting fixtures. By contrast, fluorescent lamps generally need to bemanually removed so that its lighting fixture can be cleaned.

FIGS. 15A, 15B, 15C and 15D are diagrams illustrating another embodimentof an LED lighting fixture 1500 as may be used in connection withvarious LED control network embodiments as disclosed herein. FIGS. 15Aand 15B illustrate front and back oblique views, respectively, whileFIGS. 15C and 15D illustrate cross-sectional side views of the lightingpanel illustrated in FIGS. 15A and 15B. FIG. 15D illustrates variousindividual components of the LED lighting fixture 1500 prior toassembly, whereas FIG. 15C illustrates those same components afterassembly.

As illustrated in FIGS. 15A and 15B, the lighting panel fixture 1500includes a ceiling panel 1502, an LED lighting mount 1504, and a riserpanel 1522, similar to the embodiment of FIG. 11A but without a hingemechanism. The ceiling panel 1502 is generally configured to bepositioned directly over the seating in the transit vehicle, and mayabut the interior sidewall of the transit vehicle. The riser panel 1522is, in this particular embodiment, curved in approximately a quarter arcshape, and would generally reach to the ceiling of the transit vehicleor some other top mounting surface. As before, the riser panel 1522 mayoptionally be configured to allow mounting of advertising prints orsimilar materials, as is common in the transit industry. The LEDlighting mount 1504 interconnects the ceiling panel 1502 and the riserpanel 1522. The riser panel 1522 may be secured to the LED lightingmount 1504 with screws 1543, which may be attached using clip-onextrusions as illustrated more clearly in FIGS. 15C and 15D.

The LEDs 1505 (shown in side view in FIGS. 15C and 15D) may bepositioned along the LED lighting mount 1504 in a linear array or insome other configuration, similar to the configuration illustrated inFIG. 11G. As before, a removable light permeable cover 1520 may beprovided or affixed to the LED lighting mount 1504, for softening orevening out the illumination from the LEDs 1505. Snap-in lens holder1571 may be manually removed (e.g., popped out) to allow convenientremoval of the light permeable cover 1520.

In one embodiment, the LED lighting mount 1504 of the LED lightingfixture 1500 is constructed using aluminum extrusion, and operates as aheat sink in addition to providing physical support for the LEDs. Asbefore, the LED lighting mount 1104 may be configured with metallic heatdissipating fins 1563 on the backside of one or more of its walls. Theriser panel 1522 may be constructed of a lightweight durable materialsuch as fiberglass, or PVC plastic. Ceiling panel 1502 is preferablyformed of a composite material, such as two metal layers sandwiching aninsulator material. The length of the lighting panel fixture 1500 mayvary depending upon lighting needs, and the use of LEDs (as opposed to,e.g., fluorescent tubes) increases the flexibility of fixture lengthsand sizes. In one particular example, the lighting panel fixture 1500 isapproximately 60″ in length. The lighting panel fixture 1500 maycomprise multiple LED units (or “light bars”) 1550 (in one example, fivesuch LED units 1150, each approximately 12″ long). Each LED unit 1550 inthis example may have eight LEDs 1505, arranged in a linear array, andwhich may be mounted on a metal PCB to help dissipate heat. The LEDlighting panel fixture 1500 may have a control (i.e., slave) module todrive the LEDs 1505 of the multiple LED units 1550, as described withrespect to FIGS. 2A through 4 previously. The control module may becontained in a control module housing 1560 on the backside of the LEDlighting mount 1504, as illustrated in FIGS. 15A and 15B. Multiplelighting panel fixtures 1500 may be connected using cables withcomplementary connectors at either end (similar to FIG. 2B), in a daisychain fusion, as previously described.

In the exemplary LED lighting panel fixture 1500 illustrated in FIGS.15A through 15D, the conduits in which the LED units 1550 reside areopen at either end, so that when the LED lighting panel fixtures 1500are placed in a row, a single continuous conduit is formed, therebypotentially simplifying cleaning of the lighting fixtures as previouslydescribed.

FIG. 15E is a side view diagram similar to FIGS. 15C and 15D, butillustrating a variation of the LED lighting fixture including onepossible permanent hinge mechanism. Unlike the embodiment illustrated inFIG. 11A, the riser panel 1522 in this example is not detachably hinged,but instead is permanently hinged such that it can be rotated downwards(as illustrated in FIG. 15E) to allow temporary access to, e.g., autility panel or a storage compartment. In this example, an elongatehinge member 1524, such as a “piano hinge” style hinge, traverses theriser panel 1522. Removable insert 1527 may be manually removed to allowaccess to screws 1546, which can be loosened to permit the riser panel1522 to be rotated downwards as shown in FIG. 15E. To re-secure theriser panel 1522, the opposite steps may be carried out.

In alternative embodiments, other means may be provided (such as asliding door or screw-on panel) to allow access behind the riser panel.

The control network architecture for the various embodiments of theLED-based control systems described herein may utilize, for example,control networks disclosed or described in one or more of U.S. Pat. Nos.5,907,496, 6,061,600, 6,094,416, 6,147,967, 6,201,995, 6,611,860,6,961,306, 7,046,621, 7,046,622, and/or 7,065,039, all of which areassigned to the assignee of the present invention and herebyincorporated by reference as if set forth fully herein.

For example, FIG. 5 is a high-level architectural diagram of amaster-slave control network 501 for a transit vehicle, illustrating apossible relationship of an LED-based lighting control subsystem 550 toother components in the control network 501. In the context of FIG. 5, anode acting as a master bus controller for a particular bus will bereferred to as a “master node” for that particular bus, and the othernodes on that bus will be referred to as “slave nodes” for thatparticular bus. In FIG. 5, a master node 503 and a plurality offirst-tier slave nodes 523 are connected to a main data bus 504. In apreferred embodiment, each of the slave nodes 523 is configured or canbe configured to control a secondary data bus. For example, thefirst-tier slave node 523C is shown connected to a secondary data bus523 in the control network 501. The first-tier slave node 523C functionsas a second-tier master node with respect to second-tier slave nodes 533connected to the secondary data bus 513. Others of the first-tier slavenodes 523 can also serve as second-tier master nodes and be connected todifferent secondary buses having additional second-tier slave nodes. Amulti-level or multi-tiered hierarchical control network is therebyestablished.

Each of the master node 503, first-tier slave nodes 523, second-tierslave nodes 533, and other lower-level slave nodes (not shown in FIG. 5)are referred to hereinafter generically as “nodes” and are designated asnodes 530 in FIG. 5. In one aspect of a preferred embodiment as shown inFIG. 5, each of the nodes 530 has substantially the same hardwareconfiguration and can therefore function as either a master node or aslave node, depending upon how the control network 501 is configured.Each data bus, along with the nodes attached to it, may be generallyreferred to as a cell, and the master node connected to the data bus isreferred to as a “cell controller” for that particular cell. Asexplained in more detail hereinafter, each node 530 configured as amaster node transmits and receives messages over the data bus for thecell it controls. Each node 530 configured as a slave node remains in alisten mode, receiving but not transmitting messages over that data bus,unless specifically requested to transmit information over the data busby the master node. Any number of the slave nodes can, even thoughoperating as a slave node with respect to an upper tier, besimultaneously operating as a master node with respect to otherlower-tier slave nodes at a different cell sub-level.

A preferred embodiment of the control network 501 involves a mechanismfor redundant backup control of any node functioning as a master node atany level or sub-level of the control network 501. Generally, the slavenodes connected to a particular data bus monitor the data bus while in alisten mode and await periodic signals from the master node for thatdata bus. Upon a failure to receive a signal from a master node withinan expected time, the slave nodes connected to that data bus begin await period (which is preferably a different wait period for each slavenode connected to the data bus). When the wait period elapses, the slavenode determines that a failure in the master node for the particulardata bus has occurred, and takes steps to take over the functionality ofthe master node. Each of the slave nodes is preferably programmed with adifferent wait period, so that there is no contention for replacing themaster node when a master node failure has occurred. In one aspect,backup control of each master node is prioritized, such that there is aspecific order in which the slave nodes can potentially take overcontrol of the master node functionality when a failure has occurred.

In more detail, one of the nodes 530 attached to the main data bus 504is configured as a master node 503. The other nodes 530 attached to themain data bus 504 (in this example numbering four such nodes 530) areconfigured as first-tier slave nodes 523, meaning that they receive butdo not transmit master-control signals over the main data bus 504. Thefirst-tier slave nodes 523 may, however, from time to time sendresponsive signals or status signals over the main data bus 504.

In a preferred embodiment, each of the first-tier slave nodes 523 may beconfigured as a second-tier master node controlling a secondary bus. Onesuch example is shown in FIG. 5, wherein first-tier slave node 523C isconnected to a secondary data bus 513. A plurality of other nodes 530are also attached to the secondary bus data 513, and serve assecond-tier slave nodes 533. There are three such second-tier slavenodes 533 in the example shown in FIG. 5. With respect to the secondarydata bus 513, the first-tier slave/second-tier master node 523Ctransmits master-control signals to the second-tier slave nodes 533. Thesecond-tier slave nodes 533 ordinarily operate only in a listen mode,but from time to time may send responsive messages or status messages tothe second-tier master node 523C. The other first-tier slave nodes 523A,523B and 523D may similarly be connected as second-tier master nodes(i.e., cell controllers) each controlling its own secondary bus or cell.

While the control network 501 shown in FIG. 5 has four first-tier slavenodes 523 and three second-tier slave nodes 533, the number offirst-tier slave nodes 523 and second-tier slave nodes 533 is limitedonly by the ability of the master node to communicate with the slavenodes over the particular data bus. There may be more slave nodes orfewer slave nodes on each bus than shown in the control network 501. Ina preferred embodiment, there are no more than eight such cellcontrollers, although more than eight may be used so long as processingcapacity and speed permit.

In addition, further levels of control nesting beyond two data buses mayalso be provided, using a similar approach to the two data bus method.Thus, for example, one or more of the second-tier slave nodes 533 may beconfigured as a third-tier master node controlling its own tertiary orthird-tier data bus. While FIG. 5 only shows two nested control levels,the same control concepts would apply to a control network architecturehaving additional nested control levels. Examples of control networkshaving more than two data buses are depicted in FIGS. 8 and 9 anddescribed in more detail hereinafter.

In a preferred embodiment, communication over the main data bus 504 andthe secondary data bus 513 (or buses, if appropriate) istime-multiplexed such that only one node 530 is transmitting over aparticular data bus at a given time. Usually, each transmitted messagewill be targeted for a specific destination node 530, which may bespecified by address bits in the transmitted message. However, in someembodiments broadcast messages may also be used targeted to multiplenodes 530.

Responsibilities for tasks, or groups of tasks, may be assigned tospecific nodes 530. For example, each of the first-tier slave nodes 523may be assigned a distinct sphere of responsibility. Similarly, each ofthe second-tier slave nodes 533 may be assigned a distinct sphere ofresponsibility.

Each of the nodes 530 preferably comprises an uplink transceiver 507, adownlink transceiver 508, and a switch 509. Each of the nodes 530receives signals over its downlink transceiver 508. Over the main databus 504, the first-tier master node 503 transmits master-control signalsto each of the first-tier slave nodes 523. From time to time, accordingto the programmed control protocol, the first-tier slave nodes 523respond to the master-control signals, or otherwise send status messagesto the first-tier master node 503 when events occur specific to thatfirst-tier slave node 523. Otherwise, the first-tier slave nodes 523 donot ordinarily communicate with each other.

In a similar manner, over each secondary data bus (such as secondarydata bus 513), the second-tier master node 523 (for example, first-tierslave/second-tier master node 523C in FIG. 5) transmits master-controlsignals to each of the second-tier slave nodes 533 connected to the samesecondary data bus. From time to time, according to the programmedcontrol protocol, the second-tier slave nodes 533 respond to themaster-control signals, or otherwise send status messages to thesecond-tier master node 523C when events occur specific to thatsecond-tier slave node 533. Otherwise, the second-tier slave nodes 523do not ordinarily communicate with each other.

Communication between nodes is preferably carried out using half-duplextime division multiplexing. In typical operation, the master node pollseach of the slave nodes periodically. Each of the nodes is preferablyprovided with a unique node identification number or address thatdistinguishes it from all other nodes of the control network. The masternode sends a control message to each slave unit in turn, using the nodeidentification number or address to identify the intended destination.Each of the slave nodes receives the control message but only reacts ifit recognizes its own node identification number or address in thecontrol message. The slave node takes the actions requested by thecontrol message received from the master node. Within a designated timeperiod after receiving the control message, the slave node responds tothe master node with an acknowledgment message. Each of the slave nodesare polled in turn so that the master node can keep track of eventshappening throughout the system.

A communication protocol is preferably established so as to avoidcollisions on each of the data buses. A simple and effectivecommunication protocol is one in which the master node for theparticular data bus sends a control message to a particular slave node,which responds with an acknowledgment or status message within apredetermined amount of time before the master node contacts anotherslave node. Slave nodes generally do not initiate communication withoutbeing first polled by the master node. The master node may also send outa broadcast control message that is intended for receipt by more thanone of the slave nodes. The broadcast control message can comprise anode identification number or address that instructs a single particularnode to respond to the broadcast control message. The single nodeselected for response may be the most critical node requiring receipt ofthe broadcast control message.

Failure of the current master node (at any of the control levels)commonly results in the master node either failing to transmit, or elsetransmitting improper control information to the slave nodes over thedata bus. According to a preferred redundant backup control protocol,the slave nodes periodically receive master-control messages from themaster node and, in the event that proper master-control messages failto appear, initiate a failure mode response procedure. For example, aslave node may take over the responsibilities of the master node if itfails to detect a transmission from the master node within the expectedtime period.

In one embodiment, the lighting subsystem 550 comprises a portion of thelarger control network 501 of the transit vehicle. A second-tier masternode 523B functions as the master node 203 described with respect toFIG. 2A, and the other nodes coupled to bus 513, i.e., nodes 533A, 533Band 533C in this example, function as the LED/slave nodes 202 in FIG.2A. While the lighting subsystem master node 523B generally provideshigh-level control for the lighting subsystem 550, the lightingsubsystem master node 523B may itself be a slave node with respect tothe higher level of the control network hierarchy. In other words, thelighting subsystem master node 523B may act as a slave node with respectto first-tier bus 504, under control and direction of first-tier masternode 503. In one example, the downlink transceivers 508 of slave nodes533A, 533B and 533C may not necessarily control other lower levelnetwork tiers, but rather would primarily be provided for redundantbackup control of the lighting subsystem master node 523B in accordancewith the operation described herein.

FIG. 6 is a block diagram of an embodiment of a network node 603 as maybe used within the control network architecture shown in FIG. 5.Detection of and response to a failure mode condition may be explainedin greater detail with reference to the network node 603 illustrated inFIG. 6. Because failure mode detection and response may be carried outby a node 530 operating as a slave node, the following discussion willassume that the node 603 shown in FIG. 6 is initially configured as aslave node. Further, for simplicity of explanation, it will be assumedthat the node 603 shown in FIG. 6 is a first-tier slave/second-tiermaster node connected to a main bus and a secondary bus (such asfirst-tier slave/second-tier master node 523C connected to the main databus 504 and secondary data bus 513 in FIG. 5), although the same nodecircuit configuration is preferably used for each of the nodes 530,regardless of control level, for ease of construction and flexibilitypurposes.

In the node block diagram of FIG. 6, a node 603 is shown connected to afirst bus (e.g., main bus) 604. The node 603 comprises an uplinktransceiver 611, a downlink transceiver 621, a CPU 612 connected to theuplink transceiver 611, and another CPU 622 connected to the downlinktransceiver 621. Both CPUs 612, 622 are preferably connected to adual-port RAM 618, and each CPU 612, 622 is connected to a ROM programstore 614 and 624, respectively. The second CPU 622 is connected throughan appropriate interface to I/O ports 654, which may comprise sensorinputs, control signal outputs, status LEDs, LCD display, keypad, orother types of external connections.

Each node 603 is preferably capable of both sending and receivingmessages (e.g., control instructions). Typically, the uplink transceiver611 operates in a “slave” mode whereby the node 603 receives controlinstructions using the uplink transceiver 611 and then responds thereto,and the downlink transceiver 621 operates in a “master” mode whereby thenode 603 issues control instructions (e.g., polls slave nodes) andawaits a response from other nodes after sending such controlinstructions.

The downlink transceiver 621 of the node 603 is connected to a secondarydata bus 652, to which is also connected a plurality of second-tierslave nodes 651 (assuming the node 603 is a first-tier slave/second-tiermaster node). The node 603 thereby functions as a first-tier slave nodewith respect to the main data bus 604, receiving with its uplinktransceiver 611 first-tier master-control signals over the main bus 604from a first-tier master node (such as master node 503 shown in FIG. 5),and also functions as a second-tier master node with respect to thesecondary data bus 652, transmitting second-tier master-control signalswith its downlink transceiver 634 to second-tier slave nodes 651.

The node 603 also comprises a pair of switches 635 a, 635 b connectedbetween the downlink transceiver 621 and the signal lines 643 a, 643 bof the main data bus 604. In normal operation, the switches 635 a, 635 bremain open (unless the node 503 is also the first-tier master node,such as master node 503 shown in FIG. 5, in which case the switches 635a, 635 b would be closed), and the downlink transceiver 621 is therebyisolated from the main data bus 604. However, when a first-tier masternode failure condition is detected, switches 635 a, 635 b are closed,enabling the downlink transceiver 621 to take over for the first-tiermaster node. The downlink transceiver 621 would therefore functionsimultaneously as master node with respect to both the main data bus 604and the secondary data bus 652.

In a preferred embodiment, detection of a master node failure conditionon the main data bus 604 is accomplished using a timer mechanism, suchas a hardware timer 613 accessible (either directly or indirectly) bythe CPU 612 that is connected to the uplink transceiver 611. Accordingto a preferred control protocol (assuming the node 603 is a first-tierslave/second-tier master node), the uplink transceiver 611 of node 603receives first-tier master-control signals periodically from thefirst-tier master node (such as master node 503 in FIG. 5). Themaster-control signals may, for example, request status information fromthe node 603, or instruct the node 603 to carry out certain control orinput/output functions. The node 603 ordinarily responds by carrying outthe requested functions and/or sending an acknowledgment or statussignal to the first-tier master control node using the uplinktransceiver 611.

Timer 613 times out a wait period between master-control signalsreceived from the first-tier master control node. In a preferredembodiment, each time the uplink transceiver 611 receives amaster-control signal from the first-tier master node that is recognizedas an appropriate master-control signal within the particular programmedcontrol protocol (whether or not the master-control signal is directedto the particular node 603), the CPU 612 connected to the uplinktransceiver 612 resets the timer 613. If the timer 613 ever times out,then CPU 612 responds by asserting a failure mode response procedure.The timing out of timer 613 may result in an interrupt to CPU 612 inorder to inform the CPU 612 of the failure to receive master-controlsignals, or else the CPU 612 may periodically monitor the timer 613 and,when the CPU 612 notices that the timer 613 has timed out, assert afailure mode response procedure.

When a failure mode condition is detected, the CPU 612 sets a failuremode status bit in a predetermined flag location within the dual-portRAM 618. The other CPU 622 periodically monitors the failure mode statusbit in the dual-port RAM 618 and is thereby informed when a failureoccurs. Alternatively, instead of the CPUs 612, 622 communicatingthrough the dual-port RAM 618, timer 613 can directly inform CPU 622when a failure to receive master-control signals has occurred (i.e.,when timer 613 has timed out).

When the CPU 622 has been informed or otherwise determined that afailure mode condition exists, and that the first-tier master node haspresumably failed, the CPU 622 sends a signal over control line 633 toclose switches 635 a, 635 b, thereby connecting the downlink transceiver621 to the main bus 604. From that point on, the CPU 622 performs as thefirst-tier master node with respect to the main bus 604. The node 603can continue to receive information over the main data bus 604 using theuplink transceiver 611. Alternatively, the node 603 may thereafterperform all transmission and reception over both the main bus 604 andthe secondary bus 652 using the downlink transceiver 621. When thefailure mode is entered, the CPU 622 may be programmed so as to directlycarry out the I/O port functions for which it previously receivedinstructions from the first-tier master node, or the node 603 may sendmaster-control signals to its own uplink transceiver 611 and therebycontinue to carry out the I/O port functions as it had previously beendoing. In other words, the node 603 can give itself control instructionsover the main data bus 604 so that it can continue to perform itspreviously assigned functions. If, after taking over for the first-tiermaster node, the node's downlink transceiver 611 should fail, the node603 can still continue to perform its control functions when the nextslave node takes over control as the new first-tier master node (aslater described herein), because its uplink transceiver 611 continues tofunction in a normal manner.

According to the above described technique, the node 603 therebysubstitutes itself for the first-tier master node upon the detection ofa first-tier master node failure as indicated by the failure to receivethe expected first-tier master-control signals. Should the node 603fail, either before or after taking over control for the first-tiermaster node, the next first-tier slave node would take over and becomethe first-tier master node in a similar manner to that described above.

Referring again to FIG. 5, the order in which the first-tier slave nodes523 take over for the first-tier master node 503 is dictated by the waitperiod timed out by the timer 613 of the particular first-tier slavenode 523. The timer 613 (see FIG. 6) for each first-tier slave node 523is programmed or reset using a different time-out value. A first-tierslave node 523 only asserts a failure mode condition when its internaltimer 613 reaches the particular timeout value programmed for thatparticular node 523.

While the programmed wait periods for the internal timer 613 in eachfirst-tier slave node 523 can vary depending upon the controlapplication, illustrative wait periods are programmed in ten millisecondincrements. Thus, for example, first-tier slave node 523 a could beprogrammed with a 10 millisecond wait period; the next first-tier slavenode 523 b could be programmed with a 20 millisecond wait period; thenext first-tier slave node 523 c could be programmed with a 30millisecond wait period; and the last first-tier slave node 523 d couldbe programmed with a 40 millisecond wait period; and so on. First-tierslave node 523 a would take over as the first-tier master node if 10milliseconds elapses without it receiving any proper first-tiermaster-control signals; the next first-tier slave node 523 b would takeover as the first-tier master node if 20 milliseconds elapses without itreceiving any proper first-tier master-control signals; the nextfirst-tier slave node 523 c would take over as the first-tier masternode if 30 milliseconds elapses without it receiving any properfirst-tier master-control signals; and so on.

Use of 10 millisecond increments for the wait periods in the aboveexample is considered merely illustrative, and the actual wait periodsshould be selected depending upon the time criticality of the controlmessages, and the number of messages that may be missed before a highenough degree of certainty is established that the master node hasfailed. For example, if a slave node expects to observe acontrol-message signal on the data bus no later than every 5milliseconds, then the slave node may be programmed to assert a failuremode condition after a wait period corresponding to the absence of apredefined number of messages—for example, twenty messages (i.e., 100milliseconds). If critical aspects of the system requiring master nodecontrol need to be serviced in a shorter time period, then the waitperiod would have to be reduced to accommodate the time-sensitivecomponents of the system.

The order in which the slave nodes take over for the master node neednot be dictated by the relative position in the control loop of theslave node with respect to the master node, but rather may be dictatedaccording to the programmed wait period in each slave node. Flexibilityis thereby provided in the order of priority in which the slave nodestake over for the master node in the event of a failure event.

Accordingly, by use of the aforementioned techniques described herein,redundant backup for the first-tier master node 503 is provided. Suchredundant backup control is provided without requiring additionalphysical nodes to be located within the control system, and withouthaving to provide wiring for such additional physical nodes to the buses504 or 513. The redundant backup for the master node 504 is alsoaccomplished while resolving contention problems that might otherwiseoccur if each of the first-tier slave nodes 523 were programmed with theidentical timeout period.

In a preferred embodiment, redundant backup control is provided in asimilar manner for the secondary data bus 513, and each additional databus that may be provided in the system. Thus, each of the second-tierslave nodes 533 is preferably configured with the circuitry shown fornode 603 in FIG. 6, and each of the second-tier slave nodes 533 cantherefore substitute itself for the first-tier slave/second-tier masternode 523 c if the first-tier slave/second-tier master node 523 c fails.

If a particular node is operating as a master node for two buses as aresult of a failure of the master node on a higher-tier bus, and thenode operating as such fails, then it is possible that two differentnodes will take over for the failed node, one node taking over on eachbus. For example, supposing that first-tier slave/second-tier masternode 523 c has already taken over as the first-tier master node due to afailure of the master node 503, and further suppose that first-tierslave/second-tier master node 523 c too fails, then the next first-tierslave node 523 d would take over as the first-tier master node withrespect to the main data bus 504, but the first second-tier slave node533 a would take over as second-tier master node with respect to thesecondary data bus 513.

In the above manner, despite the failure of one or more nodes,substantial functionality of the control system as a whole can bemaintained. A failed node is essentially discarded or bypassed to theextent possible so as to maintain the highest possible degree ofcontinued operability. Furthermore, because certain parts of the systemwill continue operate despite the failure of the master node,identification of the failed node by engineers or maintenance personnelshould be simplified by being able to identify the inoperative portionof the system that has become isolated due to the failure.

In one aspect, separation of responsibility in each node 603 of masterfunctions and slave functions between two different CPU's each operatingwith a different transceiver allows the node 603 to potentially continueoperating as either a master node or a slave node should one of theCPU's fail, providing that the failure does not disrupt both of thetransceivers at the node 603.

In one embodiment, the nodes 530 of FIG. 5 are wired using a singlecable connecting all of the nodes 530 in a loop configuration. Detailsof such a wiring technique are described in U.S. Pat. No. 5,907,486entitled “Wiring Method and Apparatus for Distributed Control Network,”assigned to the assignee of the present invention, and previouslyincorporated herein by reference.

In other embodiments, the nodes 530 of FIG. 5 are configured with faultisolation and recovery circuitry in the case of a short circuit orsimilar event. Details of such fault isolation and recovery circuitryare described, for example, in U.S. Pat. No. 6,147,967 entitled “FaultIsolation and Recovery In A Distributed Control Network,” incorporatedby reference as if set forth fully herein.

FIG. 7 is a diagram of another example of a control network architecturein accordance with another embodiment, and having multiple second-tierbuses. Each of the nodes shown in FIG. 7 may be configured to includethe circuitry shown for preferred node 603 in FIG. 6. FIG. 7 shows anexample of a control network architecture for a control network 701having three data buses 704, 714 and 724. A first-tier master node 703and a plurality of first-tier slave nodes 712 are connected to the maindata bus 704. One of the first-tier slave nodes 712, designated as A1 inFIG. 7, operates as a second-tier master node, and is connected to thesecond data bus 714 along with a plurality of second-tier slave nodes722. Another of the first-tier slave nodes 712, designated as D1 in FIG.7, operates as another second-tier master node, and is connected to thethird data bus 724 along with another plurality of second-tier slavenodes 732. The other first-tier slave nodes 712, designated B1 and C1 inFIG. 7, could also be configured as master nodes of a second-tier bus.The particular example in FIG. 7 thereby provides a hierarchical controlnetwork 701 having two control levels or tiers, and three data buses.

FIG. 8 is a diagram of another example of an embodiment of ahierarchical control network 801, having a third-tier bus, as may beused in connection with a lighting system control network as disclosedherein. The control network 801 shown in FIG. 8 has four buses 804, 814,824 and 834. Similar to FIG. 7, a first-tier master node 803 and aplurality of first-tier slave nodes 812 are connected to the main databus 804. One of the first-tier slave nodes 812, designated as A1 in FIG.8, operates as a second-tier master node, and is connected to the seconddata bus 814 along with a plurality of second-tier slave nodes 822.Another of the first-tier slave nodes 812, designated as D1 in FIG. 8,operates as another second-tier master node, and is connected to thethird data bus 824 along with another plurality of second-tier slavenodes 832. One of the second-tier slave nodes 832 connected to the thirddata bus 824, denoted as A2′ in FIG. 8, operates as a third-tier masternode with respect to the fourth data bus 834, which is connected to aplurality of third-tier slave nodes 842. The structure of FIG. 8 therebyprovides a hierarchical control network 801 having three control levelsor tiers, and four data buses. In the context of FIG. 8, the controlnetwork subsystem associated with any of the data buses 804, 814, 824 or834 may represent the lighting control subsystem used in a transitvehicle, although the lighting control subsystem is more likely to beassociated with a lowest level tier such as the subsystems associatedwith buses 814 or 834.

It will be appreciated that, expanding the approach used in FIGS. 7 and8, additional control levels may be created by adding successive lowercontrol tiers, or additional slave nodes at any particular tier may beconfigured as cell controllers to control additional localized databuses. A great deal of flexibility is thereby provided in establishing ahierarchical control structure suitable for many different controlapplications.

FIG. 9 is a diagram of a hierarchical control network with a matrixarchitecture providing supervision of mid- and lower-tier levels of thecontrol network, as may be used in connection with a lighting systemcontrol network as disclosed herein, and based on similar principles tothose described with respect to the control networks of FIGS. 5, 7 and8. In FIG. 9, a first-tier master node 900 and a plurality of first-tierslave nodes 902 are connected to the main data bus 901. Each of thefirst-tier slave nodes 902 (up to, e.g., k nodes) may operate as asecond-tier master node with respect to a respective second-tier databus (denoted 9S1, 9S2 . . . 9Sk in FIG. 9), each of which has connectedto it a plurality of second-tier slave nodes 903. Likewise, each ofsecond-tier slave nodes 903 may be connected to a respective third-tierdata bus (denoted 9T1, 9T2 . . . 9Tk in FIG. 9), each of which hasconnected to it a plurality of third-tier slave nodes 904. This sameexpansion may occur at lower tier levels, with respect to fourth-tierdata buses (denoted 9F1, 9F2 . . . 9Fk in FIG. 9) down to the Nth level.The control network 950 depicted in FIG. 9 thereby provides amultiple-bus hierarchical control scheme, with each node preferablycapable of fanning out to a number of lower-level slave nodes, which maybe repeated down to an arbitrary number of levels or tiers.

In a preferred embodiment, each node in FIG. 9 is given a unique nodeidentifier to facilitate communication among the various nodes in thenetwork. The unique node identifier may comprise, for example, a seriesof three binary numbers XYZ, the first number X identifying the tier forwhich the node is connected in its capacity as a slave, the secondnumber Y identifying which bus of the tier (i.e., tier X) the node isattached to by identifying the relative position of the bus with respectto the other buses attached to the tier, and the third number Zidentifying the relative position of the node with respect to othernodes attached to the same bus (i.e., bus Y of tier X).

The control network architecture depicted in FIG. 9 has the advantage ofbeing relatively easily expandable and is capable of controlling verylarge networks. To reduce the potential for time lag or latency in verylarge networks when propagating important communications from lower tiernodes through a number of intermediary nodes up to the first-tier masternode 900, it may be desirable to add a supervisory network coupled toone or more of the lower-tier buses, so as to allow monitoring ofcommunications at the lower tier levels, and to further allow thepossibility of redundant backup control and/or communication withrespect to the lower-tier master nodes.

FIGS. 10A and 10B are exemplary timing diagrams for communication in ahierarchical control network having a matrix architecture, such as thecontrol network illustrated in FIG. 9. A wide variety of othercommunication protocols may also be used, in FIG. 9 and in connectionwith other embodiments as disclosed herein.

In various embodiments, an LED-based lighting control system constructedin accordance with the principles and techniques disclosed herein mayexhibit a number of advantages and/or useful characteristics. Forexample, the lighting control system and its various components may havea long lifetime, because LEDs can operate up to 100,000 hours or more.As a result, fewer lighting fixtures require replacement over time, ascompared to, e.g., fluorescent lighting fixtures, which may reducemaintenance costs (both replacement component costs and service costs).LEDs consume less current than fluorescent lights, typically using abouthalf the power, thus resulting in a significant power savings. UsingLEDs may reduce the effects of electromagnetic coupling, and also reducearcing because the LEDs typically operate off a 12 or 24 Volt supply.

As another potential advantage, LEDs have no filaments or bulbs, and arehighly durable. LEDs are generally resistant to vibration, cold,dampness, and salt spray, providing many environmental settings in whichthe lighting systems disclosed herein may be utilized. An LED-basedlighting network may also be more environmentally friendly, as comparedwith fluorescent, incandescent and neon lighting, in that theirconstruction requires less fossil fuels (required to produce the glassneeded to encapsulate incandescent, fluorescent and neon lighting), andpotentially environmentally harmful materials (such as Mercury) or gases(such as neon) need not be used in the manufacture of LEDs.

Another advantage of the LED-based lighting systems as disclosed hereinis that the LED fixtures and individual LED units may be made anydesired length, by, for example, altering the number of LEDs in thearray. By contrast, fluorescent lights generally are available in onlycertain preset sizes, limiting flexibility. Different sized LED-basedfixtures may be mixed and matched within a transit vehicle, offeringgreat flexibility in physical layout and arrangement.

Yet another advantage of using LED-based lighting is that the LEDs canbe made more directed than incandescent or fluorescent lighting, andcan, for example, be targeted downwards towards passenger seating areas.In this manner, glare that might otherwise occur on the driver'swindshield from lighting the passenger seating compartments canadvantageously be reduced.

Use of a multiplexed control system as described in connection withcertain embodiments may enhance or facilitate control of the LED-basedlighting fixtures. Addressable IDs for the slave nodes may be used forselective control of different LED-based lighting fixtures in thelighting system. Connecting the LED-based lighting fixtures in a daisychain fashion may reduce the amount of wiring required. The LED outputlevel may also be conveniently adjusted, manually or automatically, andmay be selectively reduced for example to conserve battery output duringoverload conditions. Using the multiplexed control system, monitoringand diagnosis of the LED-based lighting fixtures is also facilitated.The LED-based lighting fixtures may also provide emergency lightingfunctionality.

In certain embodiments in which a ring loop architecture is utilized,survivability and reliability of the control network may be enhanced.Hierarchical control of lighting, in a cell-based fashion, makes thearchitecture readily scalable and relatively easy to control andprogram. The control network infrastructure provides feedback formonitoring and diagnosis, and allows automatic detection of faults andtheir location. A control network matrix architecture may also be used,in particularly complex network settings, allowing more rapid feedbackin case of emergencies.

The novel techniques described herein may be used to control bothLED-based lighting systems and conventional incandescent lights. Theprinciples described herein have applicability beyond transit vehicles,and may be used, for example, in connection with lighting systems inbuildings, factories, and other facilities. The novel techniquesdescribed herein may also be combined with more general buildinglighting control, or with air-conditioning control, environmentalcontrol, and other electronic functions (temperature, music, etc.).

While preferred embodiments of the invention have been described herein,many variations are possible which remain within the concept and scopeof the invention. Such variations would become clear to one of ordinaryskill in the art after inspection of the specification and the drawings.The invention therefore is not to be restricted except within the spiritand scope of any appended claims.

1. A lighting system for a transit vehicle, comprising: a plurality ofLED-based lighting fixtures, each of said LED-based lighting fixturescomprising a plurality of light-emitting diodes (LEDs) collectivelyproviding area lighting, and a slave node for controlling operation ofthe LEDs; a master node for controlling the slave nodes of therespective LED-based lighting fixtures and thereby controlling theoperation of said LED-based lighting fixtures; a power businterconnecting said master node and slave nodes; and a communicationbus interconnecting said master node and slave nodes; wherein each ofsaid slave nodes further comprises a voltage regulator for providing lowvoltage power to digital electronics including a slave nodemicro-controller; and wherein the slave node sets a target LED currentlevel based upon a recorded age of the LEDs, and wherein the voltageregulator associated with that slave node adjusts the amount of currentsupplied to the LEDs of the LED-based lighting fixture according to thetarget current level.
 2. The lighting system of claim 1, wherein saidLED-based lighting fixtures each further comprise an input connector andan output connector, and wherein the power bus comprises power cablesegments connecting the LED-based lighting fixtures in a daisy chain. 3.The lighting system of claim 2, wherein said communication bus isintegrated with said power cable segments, whereby the communication busis connected to said LED-based lighting fixtures in a daisy chain. 4.The lighting system of claim 1, wherein said LEDs of a particularLED-based lighting fixture are groupwise controllable via the slave nodeof the LED-based lighting fixture.
 5. The lighting system of claim 1,wherein said master node responds to a dimming control input byconveying commands over the communication bus to selectively dim LEDs atthe LED-based lighting fixtures.
 6. The lighting system of claim 5,wherein said dimming control input is provided from a manual dimmingcontrol operable by a transit vehicle driver.
 7. The lighting system ofclaim 5, wherein said dimming control input is provided from anemergency lighting command.
 8. The lighting system of claim 1, whereinsaid master node further comprises a voltage regulator for providing lowvoltage power to digital electronics including a master nodemicro-controller.
 9. The lighting system of claim 1, wherein saidvoltage regulator converts power from said power bus for use by theLED-based lighting fixture controlled by the slave node.
 10. Thelighting system of claim 1, wherein the voltage regulator adjusts a dutycycle of a pulse width modulated LED power signal in order to adjust theamount of current supplied to the LEDs of the LED-based lighting fixtureto the target current level.
 11. The lighting system of claim 10,wherein said LEDs are dimmed by reducing the duty cycle of pulse widthmodulation.
 12. The lighting system of claim 10, wherein the voltageregulator is operable to apply a duty cycle of the pulse width modulatedLED power signal below 50% when the LEDs are new and to increaseincrementally the duty cycle to greater than 90% over the lifetime ofthe LEDs.
 13. The lighting system of claim 1, wherein the LED-basedlighting fixture associated with the voltage regulator further comprisesa temperature sensor for monitoring the temperature of the LEDs of theLED-based lighting fixture, whereby said voltage regulator adjusts theamount of current supplied to the LEDs of the LED-based lighting fixtureaccording to a reading from said temperature sensor.
 14. The lightingsystem of claim 13, wherein the slave node periodically reads an outputof said temperature sensor, and commands said voltage regulator toadjust the amount of power supplied to the LEDs of the LED-basedlighting fixture in response thereto by reducing the maximum amount ofallowable current when the temperature of the LEDs increases.
 15. Thelighting system of claim 1, wherein the voltage regulator is operable toincrease incrementally the amount of current supplied to the LEDs of theLED-based lighting fixture as the recorded age of the LEDs increases.16. A lighting system for a transit vehicle, comprising: a plurality ofLED-based lighting fixtures, each of said LED-based lighting fixturescomprising a plurality of light-emitting diodes (LEDs) collectivelyproviding area lighting, and a slave node for controlling operation ofthe LEDs, wherein the slave node of at least one of said LED-basedlighting fixtures is operable to operate the LEDs in a night mode at afirst level of brightness or a daylight mode at a second level ofbrightness less than the first level of brightness; a master node forcontrolling the slave nodes of the respective LED-based lightingfixtures and thereby controlling the operation of said LED-basedlighting fixtures; a power bus interconnecting said master node andslave nodes; a communication bus interconnecting said master node andslave nodes; and an optical sensor for sensing a level of ambientillumination, whereby the LEDs of said at least one LED-based lightingfixture are automatically dimmed from the first brightness level to thesecond brightness level if the level of ambient illumination surpasses adimming command threshold.
 17. The lighting system of claim 1, whereinsaid LEDs are high output.
 18. A distributed control network for atransit vehicle lighting system, comprising: a plurality of LED-basedlighting fixtures, each of said LED-based lighting fixtures comprising aplurality of light-emitting diodes (LEDs) for illuminating an area of atransit vehicle; a plurality of slave nodes for controlling theLED-based lighting fixtures; a master node in communication with saidslave nodes; a digital communications bus for communicating among themaster node and the slave nodes; and a power bus interconnecting saidmaster node and slave nodes; wherein said power bus is integrated withthe digital communications bus in a plurality of power/data cablesegments, and wherein said master node is communicatively coupled tosaid slave nodes by connecting said slave nodes in a daisy chain usingwith said power/data cable segments; wherein each of said slave nodes isconfigured to control an LED current based upon one or both of arecorded age of the LEDs and a measured temperature of at least one ofthe LEDs.
 19. The distributed control network of claim 18, wherein therecorded age of the LEDs is stored in a lifetime counter.
 20. Thedistributed control network of claim 18, wherein the LED current isadjusted by varying a duty cycle of an input power signal across theLEDs.
 21. A distributed control network for a transit vehicle lightingsystem, comprising: a plurality of LED-based lighting fixtures, each ofsaid LED-based lighting fixtures comprising a plurality oflight-emitting diodes (LEDs) for illuminating an area of a transitvehicle; a plurality of slave nodes for controlling the LED-basedlighting fixtures; a master node in communication with said slave nodes;a digital communications bus for communicating among the master node andthe slave nodes; a power bus interconnecting said master node and slavenodes; and an optical sensor, whereby said master node operatesaccording to a plurality of brightness modes and automatically commandssaid slave nodes to dim the LEDs in conditions of brighter ambientillumination; wherein said power bus is integrated with the digitalcommunications bus in a plurality of power/data cable segments, andwherein said master node is communicatively coupled to said slave nodesby connecting said slave nodes in a daisy chain using with saidpower/data cable segments.
 22. A distributed control network for atransit vehicle lighting system, comprising: a plurality of LED-basedlighting fixtures, each of said LED-based lighting fixtures comprising aplurality of light-emitting diodes (LEDs) for illuminating an area of atransit vehicle; a plurality of slave nodes for controlling theLED-based lighting fixtures; a master node in communication with saidslave nodes; a digital communications bus for communicating among themaster node and the slave nodes; and a power bus interconnecting saidmaster node and slave nodes; wherein said power bus is integrated withthe digital communications bus in a plurality of power/data cablesegments, and wherein said master node is communicatively coupled tosaid slave nodes by connecting said slave nodes in a daisy chain usingwith said power/data cable segments; and wherein the distributed controlnetwork implements an emergency lighting mode wherein a reduced numberof LEDs are activated by one or more slave nodes.
 23. The distributedcontrol network of claim 22, further comprising a battery other than aprimary vehicle battery connected to one of more of said LED-basedlighting fixtures to provide energy as a backup power supply.
 24. AnLED-based lighting fixture for a transit vehicle, comprising: aplurality of light-emitting diodes (LEDs) for illuminating an area of atransit vehicle; a power regulator receiving an input power supplysignal; a temperature sensor for measuring a temperature of one or moreof said LEDs; and a controller for providing a target current command tosaid power regulator, said controller receiving temperature measurementsfrom said temperature sensor and adjusting the target current command inresponse thereto; wherein said controller sets the target currentcommand based at least in part upon a recorded age of the LEDs.
 25. TheLED-based lighting fixture of claim 24, wherein said power regulatoradjusts a duty cycle of pulse width modulation in order to adjust theamount of current supplied to the LEDs to the level indicated by thetarget current command.
 26. The LED-based lighting fixture of claim 25,wherein said LEDs are dimmable by reducing the duty cycle of pulse widthmodulation.
 27. The LED-based lighting fixture of claim 26, furthercomprising an optical sensor for sensing a level of ambientillumination, whereby the LEDs are automatically dimmed if the level ofambient illumination surpasses a predefined threshold.
 28. The LED-basedlighting fixture of claim 25, wherein said power regulator is operableto set the duty cycle of pulse width modulation to below 50% when theLEDs are new and to increase incrementally the duty cycle to greaterthan 90% over the lifetime of the LEDs.
 29. The LED-based lightingfixture of claim 24, wherein said controller is responsive to commandsfrom a remote master node located in the transit vehicle.
 30. TheLED-based lighting fixture of claim 29, further comprising a combinedpower/data bus for communicating between said controller and the masternode and for carrying power from a remote on-board power supply to theLED-based lighting fixture in order to provide said input power supplysignal.
 31. The LED-based lighting fixture of claim 24, furthercomprising a ceiling panel adapted to abut a transit vehicle interiorsidewall, an LED lighting mount on which said LEDs are disposed, and ariser panel adapted to abut a transit vehicle ceiling or interior topmounting surface.
 32. The LED-based lighting fixture of claim 31,wherein said lighting mount forms a conduit in which said LEDs aredisposed.
 33. The LED-based lighting fixture of claim 32, wherein saidconduit is covered by a contiguous opaque lens which partially diffusesthe LED illumination.
 34. The LED-based lighting fixture of claim 31,wherein said LEDs are disposed on a front side of said conduit, andfurther comprising a housing disposed on a back side of said lightingmount for housing control electronics including said power regulator andsaid controller, and for receiving power and data signals.
 35. TheLED-based lighting fixture of claim 24, further comprising a backupbattery power supply.
 36. The LED-based lighting fixture of claim 24,wherein said controller responds to an emergency lighting mode bycausing a reduced number of LEDs to be illuminated.
 37. The LED-basedlighting fixture of claim 36, wherein said reduced number of LEDs arecapable of drawing power from a backup battery power supply when in saidemergency lighting mode.
 38. The LED-based lighting fixture of claim 24,wherein said controller is operable to increase incrementally the amountof current supplied to the LEDs as the recorded age of the LEDsincreases.
 39. An LED-based lighting fixture for a transit vehicle,comprising: a plurality of light-emitting diodes (LEDs) for illuminatingan area of a transit vehicle; a power regulator receiving an input powersupply signal; a temperature sensor for measuring a temperature of oneor more of said LEDs; a controller for providing a target currentcommand to said power regulator, said controller receiving temperaturemeasurements from said temperature sensor and adjusting the targetcurrent command in response thereto; a ceiling panel adapted to abut atransit vehicle interior sidewall, an LED lighting mount on which saidLEDs are disposed; and a riser panel adapted to abut a transit vehicleceiling or interior top mounting surface; wherein said riser panelincludes a locking mechanism for securing the riser panel to the transitvehicle ceiling or interior top mounting surface, and a detachable hingefor releasably attaching to said LED lighting mount, whereby the riserpanel may be fully removed from and reattached in position by releasingsaid locking mechanism.
 40. An LED-based lighting fixture for a transitvehicle, comprising: a plurality of light-emitting diodes (LEDs) forilluminating an area of a transit vehicle; a power regulator receivingan input power supply signal; a temperature sensor for measuring atemperature of one or more of said LEDs; a controller for providing atarget current command to said power regulator, said controllerreceiving temperature measurements from said temperature sensor andadjusting the target current command in response thereto; a ceilingpanel adapted to abut a transit vehicle interior sidewall, an LEDlighting mount on which said LEDs are disposed; and a riser paneladapted to abut a transit vehicle ceiling or interior top mountingsurface; wherein said riser panel includes a locking mechanism forsecuring the riser panel to the transit vehicle ceiling or interior topmounting surface, and a hinge for attaching the riser panel to said LEDlighting mount, whereby the riser panel may be rotationally swung openby releasing said locking mechanism.